Liquid-Ice Coexistence below the Melting Temperature for Water Confined in Hydrophilic and Hydrophobic Nanopores

We use molecular dynamics simulations to investigate the coexistence between confined ice and liquid water as a function of temperature for a series of cylindrical nanopores with water–wall interactions ranging from strongly hydrophilic to very hydrophobic. In agreement with previous results from experiments, we find that the ice formed in the nanopores is a hybrid ice I with stacks of cubic and hexagonal layers and that the melting temperature of the nanoconfined ice is strongly dependent on the radius of the pore but rather insensitive to the hydrophilicity of the pore surface. We find a premelted liquid layer in coexistence with the confined ice down to the lowest temperatures of this study, 50 K below the melting temperatures of the confined ices. The fraction of water in the premelted liquid layer decreases with increasing hydrophobicity of the pore wall, but it does not vanish even for the most hydrophobic nanopores. The simulations suggest that the decrease in the fraction of water in the liquid layer with increasing hydrophobicity corresponds partly to a decrease in its width but also to a decrease in its effective density: the premelted liquid layer becomes sparser on decreasing the water–wall attraction. Our results indicate that agreement in the melting temperatures of water nanopores functionalized with different moieties does not imply identical fraction of nonfreezable water in these materials.